Fuel lines of polyamide 11 and 12 have been fitted in motor vehicles for a long time, but suffer from the disadvantage that considerable permeation of conventional fuels through the walls of such lines occurs. This is particularly undesirable due to the requirements of environmental protection and safety which have arisen in recent years.
A further disadvantage of such tubing resides in the considerable absorption capacity of the polymers toward individual components of the fuels; this can lead to swelling and changes of length in the walls or wall layers of the tubing. Different degrees of swelling in different wall layers are particularly detrimental.
Developments have therefore taken place in order to improve the so-called mono-pipes consisting of a single homogeneous layer; e.g. polyamide 11 or polyamide 12. One possible improvement resides in the transition to multi-layered tubes with special barrier layers of polymers.
A fuel line in which ethylene/vinyl alcohol copolymers are associated with polyamide layers is known from DE 35 10 395 C2. However, the adhesion between these layers is so slight that they delaminate easily. Corrosive chemicals, such as scattered salt, can penetrate between the layers at the delaminated pipe ends. Furthermore, the adhesion at fittings having mandrel profiles is greatly reduced. In addition, the cold impact resistance of such pipes is so low that they cannot withstand cold impact tests according ISO 7628 and SAE J 844d, because the extremely brittle polyethylene vinyl alcohol layer has been shifted inward in the tubing wall.
DE 38 27 092 C1 describes a fuel line in which thermoplastic polyester elastomers are combined with polyamide 6 and a polyethylene vinyl alcohol internal layer. In this case also there is only slight adhesion between the layers, so the disadvantages described hereinbefore also arise.